Add spec to make sure it compiles without warnings

Make it only run with an ENV var present and make it run in a separate CI check so it can be optional. But at least then it’s red and we can decide if we need to do something about it. Should catch issues like #529

Review the repository's existing specs and CI configuration, then determine how compilation-with-warnings checks are currently run. Make the check conditional on an environment variable and place it in a separate optional CI check; completion means it can detect problems like issue #529 without affecting the normal checks.

Written by the indexing model from the issue text.